[SPARK-3143][MLLIB] add tf-idf user guide

Moved TF-IDF before Word2Vec because the former is more basic. I also added a link for Word2Vec. @atalwalkar
"This approach avoids the need to compute a global term-to-index map, which can be expensive for a large corpus,
but it suffers from potential hash collisions, where different raw features may become the same term after hashing."